The present invention relates to a process for short-time arc-welding elements, such as for example metal studs, on to components, such as for example metal sheets, wherein an element in the context of a welding operation is initially raised in relation to the component, a welding arc is formed between the element and the component, and the element is lowered again, the lifting height of the element in relation to the component being detected at least within a time segment between raising and lowering of the component.
The present invention further relates to a short-time arc-welding system for welding elements, such as for example metal studs, on to components, such as for example metal sheets, with a welding device which, in the context of a welding operation, initially raises an element in relation to the component and then lowers said element again, to a lifting height detection device to detect the lifting height of the element in relation to the component at least within a time segment between raising and lowering of the component, to a power supply device which provides the power for forming an arc between the component and the raised element, and to a control device which triggers the welding device to raise and lower the element.
This process of short-time arc-welding or this short-time arc-welding system respectively is also known generally by the term “stud welding”, for example from a brochure entitled “Die neue TUCKER-Technologie. Bolzenschweiβen mit System!” [“New TUCKER technology. The stud welding system!”], published by Emhart TUCKER in September 1999.
The technology of stud welding is used in particular, but not exclusively, in automotive engineering. With this technology, studs with or without a thread, nuts, eyes and other elements can be welded on to body panels. As a rule, the elements then serve as holding anchors for fastening interior trim elements, for example, to the vehicle body.
In the case of the above cited stud welding in accordance with TUCKER, an element is initially placed in a welding head. This can take place using an automatic feed device, for example by means of compressed air. By means of the welding head the element is now positioned at the appropriate point on the component. A pre-welding current is then switched on which flows through the stud and the component. The element is then raised in relation to the component. An arc forms. The arc flow is selected such that initially contaminants, surface coatings such as zinc, oil or dry-film lubricants, etc. are burned off. The current then switches to a welding current. As a result of the high welding current the mutually opposing end faces of the element and component melt. The element is then lowered again on to the component so the mutual melts are mixed. The welding current is switched off when the component is reached and the arc short-circuits. The melt solidifies and the weld joint is complete.
In a generally known embodiment, the element is raised and lowered in relation to the component takes place by means of an electromagnet or by means of a spring. To raise the element, the electromagnet is triggered against the force of a compression spring. To lower the element, the electromagnet is switched off and the energy stored in the spring presses the element against the component. In a more recent embodiment, the raising and lowering of the element in relation to the component are achieved by an electric motor, in particular by a linear motor. The linear motor is capable of driving along a predetermined lifting profile over time highly dynamically. In order to ensure that the specified lifting profile over time does not alter during operation, it is known to measure the path of the linear motor and to return said path to form a closed loop.
From EP 0241 249 B1 (corresponding to DE-OS 36 11 823), it is known to measure the welding arc voltage and to control the welding current provided by means of a switched-mode power supply as a factor of the detected arc voltage, it thus being possible to even out the welding conditions varying from welding point to welding point and, despite unfavorable circumstances, to achieve perfect welds. A fixed lifting curve over time is predetermined for the lifting movements of the element in relation to the component. A further short-time arc-welding system is known from WO 96/10468. In this system the arc voltage during the welding operation is regulated to a desired value wherein the lifting height of the element in relation to the component is regulated. By this means it is possible not only to regulate the welding current but also the welding voltage during the welding operation, such that predetermined profiles of these parameters are repeatedly optimally achieved over many welding operations. The lowering operation takes place at a predetermined lowering speed and is initiated at a certain point in time. It has been shown that the above-mentioned known short-time arc-welding processes are still capable of improvement in respect of consistency from welding operation to welding operation. The object of the present invention is to indicate an improved process of short-time arc-welding or an improved short-time arc-welding system, with which process or system, irrespective of the particular welding conditions, optimal welding results are achieved. The object of the invention is achieved in the case of the process of short-time arc-welding mentioned at the outset in that the initiation of the lowering operation and/or the lowering operation itself is controlled as a factor of the detected lifting height such that a previously specified total welding time is achieved. In the short-time arc-welding system mentioned at the outset, the object of the invention is achieved in that the control device triggers the welding device in such a manner that the initiation of the lowering operation and/or the lowering operation itself takes place as a factor of the lifting height reached such that a previously specified total welding time is achieved. According to a further aspect, this object in the case of the process of short-time arc-welding mentioned at the outset is achieved in that the initiation of the lowering operation and/or the lowering operation itself is controlled as a factor of the lifting height detected, such that a previously specified total welding energy is achieved. The object of the invention is fully achieved in this manner. In the case of systems, in particular, wherein the lifting height during the welding operation is variable, the present invention offers the advantage of a more even quality of weld. In the case of the arc-welding process with regulation of the lifting height according to the prior art, owing to the fixed point in time of the initiation of the lowering operation and owing to the fixed prescription of the lowering speed, different total welding times may result. This can lead to different welding results from case to case. By contrast, according to the invention the previously specified total welding time is always reached constantly from case to case, irrespective of the lifting height regulation curve. Altogether, more consistent welding results can be achieved in this manner despite different boundary conditions (for example surface states).
In accordance with a further aspect of the invention, to achieve more consistent welding results there is no focus on a previously specified total welding time, but instead on a previously specified total welding energy, wherein the energy input into the welding operation is predetermined and the point in time of initiation of the lowering operation and/or the lowering operation itself are controlled as a factor of the detected lifting height such that the previously specified total welding energy is achieved.
The description hereinafter generally focuses on total welding time. All references to total welding time, however, are also intended to refer alternatively or cumulatively to the total welding energy. For example, it may be sensible to control the initiation of the lowering operation and/or the lowering operation itself as a factor of the detected lifting height such that a previously specified combination of total welding time and total welding energy is achieved.
The present invention is also applicable to arc-welding processes wherein the lifting height is not regulated, but is controlled to a value which is substantially constant. When using the welding process according to the invention, it is possible in this embodiment to achieve a previously specified total welding time without necessitating the prescription of a specific trigger time for initiating the lowering operation. Instead, the process according to the invention is used to achieve the previously specified total welding time by detecting the lifting height during the welding operation and controlling the initiation of the lowering operation such that the previously specified total welding time is achieved. In a particularly preferred process the point in time of initiation of the lowering operation and/or the lowering speed is/are controlled as a factor of the detected lifting height in order to achieve the previously specified total welding time. By means of these two parameters, the previously specified total welding time can be achieved in a comparatively simple manner in software engineering terms. It is especially preferred if the lowering speed is constant irrespective of the lifting height and if the time of initiating the lowering operation is controlled as a factor of the lifting height in order to achieve the previously specified total welding time. In this embodiment the lowering speed can be selected such that, on the one hand, the lowering operation takes place rapidly enough to prevent melt dripping from the underside of the element. On the other hand, a speed can be selected that is sufficiently slow in order to prevent excessively rapid immersion in the melt of the component and thus rebounding. Depending on the actual lifting height during the welding operation, the lowering operation is initiated such that the previously specified total welding time is achieved.
In accordance with an alternative embodiment, the lowering operation is initiated at a previously specified time and the lowering speed is controlled in order to achieve the previously specified total welding time. This alternative embodiment is slightly easier to produce in terms of programming. The lowering speed is optionally variable, however, depending on the respective lifting height regulated during the welding operation. From the foregoing it is understood that the goal of achieving a previously specified total welding time can also be achieved in that control of the initiation of the lowering operation and control of the lowering operation itself (its speed) can be combined with one another.
Furthermore, it is also possible to control the lowering operation itself so as to control the acceleration of the lowering movement. For example, it may be sensible to lower the element at high speed directly after initiating the lowering operation and to reduce said speed gradually in order subsequently to achieve a comparatively gentle immersion of the element in the melt of the component. Altogether it is preferable if the lifting height during the welding operation is regulated at least until the lowering operation is initiated. It is especially preferable if the regulation of the lifting height until initiation of the lowering operation serves to regulate the voltage of the welding arc to a constant value.
In the short-time arc-welding system according to the invention it is advantageous if the welding device has an electric motor to raise and lower the element. Although generally the present invention is also applicable to welding devices which use a combination of an electromagnet and a spring for lifting movements of the element, the use of an electric motor is preferred because of the ease of regulating electric motors and the comparatively simply achieved damping possibilities. In this context it is of particular advantage if the motor is a linear motor. In this development, it is not necessary to provide a rotational-translational converter to convert the rotational movements of the conventional electric motor into linear movements of the component. It is understood that the previously mentioned features and the features to be explained hereinafter can be used not only in the respectively indicated combination, but also in other combinations, or in isolation, without going beyond the scope of the present invention.